1. Field of the Invention
This invention relates to a surface-acoustic-wave parametric device for use as a variable-frequency selecting device. More particularly, this invention relates to a surface-acoustic-wave parametric device wherein a parametric interaction region has a form corresponding to a desired output frequency characteristic, thereby enabling design of a frequency characteristic of the variable-frequency selecting device to be made as desired.
2. Description of the Prior Art
One of the inventors of the present invention has already disclosed, in Japanese Laying-Open No. 54-41089 (1979), a surface-acoustic-wave device having a variable frequency selecting function as illustrated in FIG. 1.
In FIG. 1, numeral 1 designates a semiconductor substrate, and an insulator film 2 and a piezoelectric layer 3 are laminated on the semiconductor substrate 1. A square pumping electrode 4 to which a DC bias voltage and a pumping voltage are applied, and input and output transducers 5 and 6 are arranged on the piezoelectric layer 3.
Numeral 7 designates a DC power source for applying a DC bias voltage, 8 designates an inductor for AC blocking, 9 designates a high-frequency power source for applying a pumping voltage, 10 is a capacitor for DC blocking, and 11 and 12 designate surface-acoustic-wave absorbing members for preventing undesired reflection of surface acoustic wave at the ends of the device.
The DC bias voltage is applied from the DC power source 7 to the pumping electrode 4 so as to create a suitable depletion-layer capacitance at a surface portion of the semiconductor substrate 1 under the pumping electrode 4. Further, the pumping voltage having a frequency 2fo twice that of a center frequency fo of a desired frequency band is applied from the high-frequency power source 9 to the pumping electrode 4 so that the depletion layer capacitance is caused to oscillate and is modulated at the frequency 2fo.
When an electric signal is applied to the broad-band input transducer 5, the input electric signal is converted into a surface-acoustic-wave signal which is propagated on the surface of the piezoelectric layer 3 rightwardly and leftwardly as viewed in FIG. 1.
In the course that a signal component of the surface-acoustic-wave input signal 13 propagating in the rightward direction, which has a frequency around fo, passes through an operating region under the pumping electrode 4, the piezoelectric potential is subjected to a parametric interaction with the pumping voltage due to the depletion layer capacitance non-linearity effect on the surface of the semiconductor substrate 1 and the component is amplified. This amplified surface-acoustic-wave signal 14 is converted into and outputted in the form of an electric signal from the coutput transducer 6.
At the same time, a surface-acoustic-wave signal 15, which has a frequency fi (fi=2fo-fs, fs:a frequency of the input signal corresponding to the amplitude of the surface-acoustic-wave input signal 13, is also produced from the pumping electrode 4 and propagated leftwardly as viewed in FIG. 1. This surface-acoustic-wave signal 15 may also be outputted as an output signal.
The frequency characteristics 14a, 15a, 14b and 15b of the respective output surface-acoustic-wave signals 14 and 15 are shown, in FIGS. 2 and 3, in relation with the input signal 13 whose amplitude is shown as 1 in the figures. FIG. 2 shows the case where the pumping voltage is relatively small and FIG. 3 shows the case where the pumping voltage is relatively large.
As apparent from FIGS. 2 and 3, in the surface-acoustic-wave device having a square pumping electrode, a response at a signal passing band and a spurious response are substantially determined when an output at a desired center frequency fo is selected. By this reason, when the conventional surface-acoustic-wave device is used as a frequency selecting device, the frequency characteristic cannot be designed freely. And yet, the spurious response is still too high to be practically used.